The invention relates to optical inspection of smooth substrates having a film coating, such as silicon wafers having a dielectric film coating, in order to detect defects in the film. The invention relates more particularly to a method for discriminating between holes in the film and particles on the film.
Progress in perfecting the basic elements of advanced computers is accomplished largely by miniaturization of the microchip structures. It follows that the standards for manufacturing of microprocessors and hard disks become more and more demanding. The production of defect-free silicon wafers is a primary goal of the semiconductor industry. High-yield manufacturing of the wafers requires decreasing both the size and fractional number of defects. As many of the wafer defects that can cause problems in manufacturing of integrated circuits cannot be visually detected, automated optical surface scanners are used for non-destructive inspection of the wafers. Optical scanners typically include a laser source of light and a system of collectors to capture the light scattered by micro defects in or on the surface being inspected. Refinement of the surface scanner design represents a prime concern for the companies developing the metrology equipment for the semiconductor industry. A lot of effort has been reported on the problem of detecting, sizing, and mapping such micro defects as surface pits and contaminating particles.
One of the less-investigated defects is a hole in a film deposited at a smooth surface of a silicone substrate. In this connection, a practical problem is the ability to distinguish a hole in a film, arising from a disturbance in the process of its manufacturing, from a contaminating particle resting on the film surface. It can be difficult to properly design and conduct experimental analyses of the light-scattering properties of film holes and particles. Accordingly, the inventors have endeavored to conduct computer simulations of the light-scattering properties of micro defects in and on plane-layered structures such as filmed silicon wafers.
The Discrete Sources Method (DSM) is an efficient and flexible tool for computer simulation of light scattering by an axisymmetric structure. The essence of the DSM consists of starting with an approximate solution of the scattering problem by representing it as a finite linear combination of the fields of dipoles and multipoles, deposited at some complementary domain. The solution constructed satisfies the system of Maxwell""s equations everywhere outside medium discontinuity and infinity conditions. The amplitudes of the Discrete Sources (DS) are to be determined from boundary conditions enforced at the surface of the local obstacle causing the scattering. Thus, the scattering problem is reduced to the approximation problem of an exciting field at the obstacle surface by DS fields. A completeness of a DS fields system provides a convergence of the approximate solution to the exact one.
In prior studies, the DSM analysis has been applied to the problem of scattering of P- and S-polarized light by particles and pits on a bare silicone substrate. See, for example, Eremin, Yu. A., Orlov, N. V., Simulation of light Scattering from Particle upon Wafer Surface, Appl. Opt. 35 (1996) 33, 6599-6605; Eremin, Yu. A., Orlov, N. V., Study of Scattering Properties of Defects of Silicon Wafers, Opt. Spectr. 84 (1998) 4, 557-562. However, there is no known analysis providing a method for discriminating between particles resting on a film and holes formed in a film on a substrate.
The present invention is a result of an extension of the DSM analysis to the case of polarized light scattering by a particle located on a film or a hole formed in the film deposited on a substrate. The DSM analyses performed by the inventors suggest a convenient and efficient method and apparatus enabling optical surface scanning of a filmed substrate such as a filmed silicon wafer in order to discriminate between particles on and holes in a film. In accordance with a preferred embodiment of the invention, light that is P-polarized or at least has a strong P-polarized component is directed onto the filmed substrate at two (or more) different incidence angles, one angle being relatively large and the other angle being relatively small as measured from a surface normal. Light that is scattered into a back region of the hemispherical space above the substrate surface (i.e., scattered generally back toward the direction from which the incident beam approaches the surface) is collected and the intensity of the collected light is measured for each of the two incident angles. It has been found through DSM modeling that for holes formed in the film, the back-scattered light intensity at a relatively large incidence (i.e., highly oblique incidence) is substantially smaller than the back-scattered light intensity at a relatively small incidence (i.e., normal or near normal incidence). For particles, however, there is no such substantial decrease in intensity, and in many cases the intensity actually increases slightly from small to large incidence. Accordingly, a defect can be classified as either a hole or a particle by scanning the surface at both small and large incidence, measuring back-scattered light intensity for both incidence angles, and looking for a substantial decrease in intensity. If there is no such decrease, the defect is a particle; if there is a substantial decrease, the defect is a hole.
A suitable apparatus for carrying out the method of the present invention can include various types of collector geometries for detecting the intensity of light scattered into a back region of the hemisphere. The range of scattering angles over which collection of light was modeled by the DSM technique was xe2x88x9220xc2x0 to xe2x88x9275xc2x0, and the range of azimuth angles was xe2x88x92177xc2x0 to xe2x88x92120xc2x0 and 120xc2x0 to 177xc2x0 (zero azimuth angle being defined as lying on the intersection of the incident plane with the substrate surface in the forward direction). Advantageously, in practice a semi-annular collector can be used to collect light scattered over a semi-annular portion (i.e., covering azimuth angles of about 90xc2x0 to xe2x88x9290xc2x0) of the back half of the hemisphere and extending over a wide range of scattering angles. However, any configuration of one or more collectors capable of collecting light scattered over a relatively wide range of scattering angles and azimuth angles can be used.
The extension of the DSM technique developed by the inventors provides an opportunity to realize effective code allowing computer simulations of the light-scattering properties of film holes and particles to be readily performed. This extension of the technique in turn has led to the development of methods and apparatus that can be used to conveniently and efficiently discriminate between holes and particles on a filmed substrate.